As is well known in the art, in electrophotographic methods, there has been generally adopted the method in which a photoreceptor formed of a photoconductive material such as selenium, OPC (organic semiconductor), a-Si or the like is used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush development method or the like, a toner that is charged into a polarity reverse to that of the latent image is attached onto the latent image by an electrostatic force to develop the latent image.
In the above developing process, there is used a two-component system developer comprising a toner and a carrier. The carrying particles called a magnetic carrier act for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and also act for transferring the toner through a developing sleeve accommodating magnets therein into a developing zone near the surface of the photoreceptor on which the latent image is formed, by using a magnetic force of the magnets.
The electrophotographic methods have been widely applied to copying machines and printers. In recent years, in the market, there is an increasing demand for electrophotographic images having a much higher stability and quality. In order to meet the requirement for high-quality images, it is considered to be effective to reduce a particle size of the carrier. For this reason, various small-size carriers have been proposed. The small-size carriers are capable of forming a dense magnetic brush with bristles having a good flowability, and therefore hardly suffer from occurrence of traces of the bristles on images produced, etc. However, with the reduction in particle size of the carrier, individual carrier particles have a reduced magnetization, so that a constraint force of the magnetic carrier on a developing sleeve tends to become small. As a result, a so-called carrier adhesion phenomenon in which the carrier is transferred from a developer carrying member to a photoreceptor to thereby produce defective images tends to be readily caused.
Further, since the small-size carrier is unlikely to cause frictional electrification with a toner because of a poor flowability thereof, there has been proposed the method in which a toner and a carrier are stirred and mixed with each other with an enhanced agitation intensity. However, the enhanced agitation intensity tends to cause increase in stress exerted on the developer, so that there tends to occur a so-called spent toner phenomenon in which the toner is adhered onto a surface of the carrier. As a result, there tends to arise such a problem that deterioration in properties of the developer is promoted, and it is not possible to maintain good properties of the developer for a long period of time.
With the market's requirements such as personalization and space saving, reduction in size of the electrophotographic image-forming apparatuses such as copying machines and printers has been promoted. Further, with the reduction in size of these apparatuses, reduction in size of respective units used in the apparatuses have also been promoted, so that it is required to stably maintain properties of the developer even when used in such a small-size developing device, i.e., even when using the developer in a small amount.
In general, in order to reduce power consumption in small size apparatuses, there is a demand for a toner that is capable of sufficiently fixing images with a low fixing energy, i.e., a so-called low-temperature fixing toner. In the case of the toners that can ensure a good fixing property at a low temperature by using a low-molecular weight resin therein, etc., it is possible to achieve saving of energy. However, when subjected to repeated development a plurality of times for a long period of time, the toners tend to be spent on a surface of the carrier during continuous use under high-temperature and high-humidity conditions owing to heat or pressure generated thereupon, or the carrier particles tend to be strongly coagulated together such that the toner is entangled between the spent portions, so that there tends to arise such a phenomenon that the developer suffers from blocking, etc. As a result, variation in frictional electric charge amount of the developer tends to occur, thereby causing variation in image density and occurrence of fogging.
In order to prevent occurrence of spent toner onto the surface of the carrier, there has been conventionally proposed the method in which the surface of the carrier is coated with various resins. For example, it is known that the surface of the respective carrier core particles is coated with a releasable resin such as a fluororesin and a silicone resin. Such a coated carrier hardly suffers from occurrence of spent toner upon the development because the surface thereof is coated with the low-surface energy material. As a result, the carrier has a stable electric charge amount, and the developer using the carrier exhibits a long service life.
On the other hand, since the resin-coated carrier is in the form of an insulating material, the carrier hardly acts as a developing electrode, thereby causing such a phenomenon as referred to as an edge effect, in particular, at solid image portions. In addition, the developing bias tends to become large, so that there tends to occur carrier adhesion on non-image portions.
In order to solve the above problems, there has been proposed the method of adjusting an electric resistance value of a coating layer by dispersing a conductive material in the coating layer. However, even though an initial electric resistance value of the coating layer of the carrier is adjusted by the above method, the coating layer tends to be abraded and reduced by friction, falling-off, etc., owing to stirring in the developing device when used for a long period of time, so that if the core material is a conductive material having a low dielectric breakdown voltage, there occurs a leakage phenomenon owing to exposure of the core material to outside, thereby causing such a problem that the electric resistance value of the carrier is gradually decreased and the carrier is deposited on image-forming regions.
In general, in the case where carbon black or the like as the above conductive material is dispersed in the coating layer, the increase in amount of carbon black added tends to cause decrease in electric resistance value of the carrier. However, it may be difficult to prepare a carrier whose electric resistance value lies in a medium range of 108 to 1012 Ωcm by varying the amount of carbon black added to the coating layer.
Also, the magnetic carrier of a resin-coated type exhibits a high electric resistance value when a voltage applied thereto is low. However, when applying a high voltage to the magnetic carrier, there tends to occur leakage of electric charges therefrom owing to adverse influence of a core material thereof by itself. In particular, when a low-electrical resistance material such as an iron powder and magnetite is used as the core material, the above tendency tends to become more remarkable. Thus, when the electric resistance value of the carrier has a large voltage dependency, the resulting images tend to be generally deteriorated in gradation.
Hitherto, as the carrier constituting a two-component system developer, there are known an iron powder carrier, a ferrite carrier and a magnetic material-dispersed carrier prepared by dispersing magnetic particles in a binder resin.
The iron powder carrier and ferrite carrier are usually used in the form of resin-coated particles. However, since the iron powder carrier has a true specific gravity as large as 7 to 8 g/cm3, whereas the ferrite carrier has a true specific gravity as large as 4.5 to 5.5 g/cm3. Therefore, a large driving force is required to stir these carriers in a developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as deterioration in charging property of the carrier itself, and facilitated damage to a photoreceptor. Further, since the adhesion between the surface of the respective particles and the coating resin is not so good, the coating resin tends to be gradually peeled off during use with time, thereby causing variation in a charging property of the carrier. As a result, the problems such as formation of image defect and carrier adhesion tend to be caused.
The carriers of a magnetic material-dispersed type comprising spherical composite particles constituted of magnetic particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 and Japanese Patent Application Laid-Open (KOKAI) No. 8-6303 have a true specific gravity of 3 to 4 g/cm3 which is smaller than those of the above iron powder carrier and ferrite carrier, so that an energy upon impingement between the toner and carrier tends to be reduced, thereby advantageously avoiding occurrence of spent toner. Further, these carriers are far excellent in adhesion to coating resins as compared to the iron powder carrier or ferrite carrier and, therefore, hardly suffers from the problem that the coating resin is peeled-off therefrom during use.
However, with the recent wide spread of digital copying machines and laser beam printers using a reversal development method, it has been required that the carrier has not only a high dielectric breakdown voltage owing to application of a high bias voltage thereto in the method, but also provides a developed image having a high image density and a high quality with a good gradation, etc. Therefore, the carrier is required to have a long service life capable of maintaining various properties such as charging characteristics and electric resistance for a long period of time as compared to the conventional carriers.
Further, there have been attempted several methods in which composite particles comprising ferromagnetic iron oxide fine particles and a cured phenol resin are used as a magnetic carrier for an electrophotographic developer. For example, there are known the technology of coating a surface of respective composite core particles comprising ferromagnetic fine particles and a cured phenol resin with a melamine resin to increase an electric resistance value thereof (Patent Literature 1); the technology of forming a coating layer comprising a cured copolymer resin obtained from at least one resin selected from the group consisting of a melamine resin, an aniline resin and a urea resin, and a phenol resin, on a surface of respective composite core particles comprising iron oxide particles and a cured phenol resin to control an electric resistance value of a carrier (Patent Literature 2); the magnetic carrier comprising carrier core particles comprising ferromagnetic compound particles, non-magnetic inorganic compound particles and a phenol resin, and a nitrogen compound-containing or -bonding layer formed on the surface of the respective carrier core particles (Patent Literature 3); the carrier comprising core material particles comprising magnetic particles and a binder resin, and a first resin coating layer comprising a nitrogen-containing resin and a second resin coating layer comprising conductive particles which layers are formed on the surface of the respective core material particles (Patent Literature 4); or the like.
As typical examples of recent technologies for suppressing carrier adhesion, there are known the technology of defining a volume average particle diameter, a particle size distribution, an mean porosity and a magnetization value of a core material of the carrier as well as a difference in magnetization of the core material from that of scattered materials (Patent Literature 5), the technology of defining various properties of magnetic carrier particles comprising at least a binder resin and magnetic metal oxide particles, such as a number average particle diameter, a resistivity when applying a voltage of 25 to 500 V thereto, a true specific gravity, a magnetization intensity and a content of Fe(II) based on a concentration of an eluted iron element on a surface thereof (Patent Literature 6), the technology of defining a magnetization intensity of each of a resin carrier A having a specific average particle diameter and a resin carrier B comprising a specific amount of particles having a particle size of not more than 20 μm as measured by a mesh method, as well as a difference in magnetization between the carrier A and the carrier B (Patent Literature 7), and the like.